Many different inverter topologies are known from the prior art. A very common embodiment example is the so-called H-bridge in which four semiconductor switches, in particular IGBTs, are disposed in a bridge circuit configuration. The switching semiconductor switches are generally PWM-controlled to simulate a sinusoidal voltage waveform of a connected AC system, the switching frequency being a multiple of the system frequency.
Inverters are being increasingly used for feeding power from alternative energy sources, such as solar generators or fuel cells, into public utility grids or off-grid systems. To make this form of power generation cost-effective, all the components must have high levels of efficiency. This applies particularly to inverters for grid-synchronous conversion of the power generated.
JP 2001 320 884 A or JP 2006 197 711 A, for example, describe inverters comprising H-bridges, the semiconductor switches of which operate powerlessly, thereby minimizing total power dissipation. Known techniques here are zero voltage switching (ZVS) and zero current switching (ZCS).
Another source of power loss are the freewheeling phases after a semiconductor switch has turned off. These are caused by the output-side choke circuits whose current continues to flow when the semiconductor switch is OFF. Freewheeling paths within an inverter circuit are mainly formed via parasitic diodes of the semiconductor switches or via freewheeling diodes specially provided for the purpose. Since a voltage is generally dropped across these diodes at the start of a freewheeling phase, as the current through the diodes increases, power dissipation occurs which places an upper limit on the efficiency of the inverter.